Oracle Multi-Table association How to update multiple fields

Note the point:
1, the Update main table must be added filter conditions.
2, the query to update the result set, but also to be updated as the main table conditions, as the synchronization of everyone is to update this part of the data.
Update Student Stu
Set (Stu.name,stu.sex) =
(Select Bak.name,bak.sex
From Student_bak Bak
where bak.id = Stu.id
and stu.age = 20
)
where exists (select 1
From Student_bak Bak
where bak.id = Stu.id
and stu.age = 20
)

Oracle Multi-Table association How to update multiple fields